1. Field of the Invention
This invention relates to the field of turbochargers for use with internal combustion engines. More particularly, the invention pertains to the exhaust gas admission system of a turbocharger and to systems for controlling the exhaust gas admission.
2. Description of the Prior Art
A turbocharger for use with an automotive engine generally includes a compressor rotor mounted on one end of a driveshaft and a turbine rotor mounted on the shaft at the opposite end. The air-fuel mixture enters the compressor from the carburetor, is compressed, and passed through the intake manifold of the engine. The exhaust gas produced by the engine has substantially greater volume, temperature and energy than the inducted air. The turbine converts the energy of the exhaust gas to drive the compressor, which boosts the pressure of the air-fuel mixture before it is inducted by the engine. In this way, a turbocharged engine will have a greater mass of air delivered to the engine than will a naturally aspirated system, more fuel can be burned and more power can be produced by the engine. The increase in power is used to accelerate the vehicle at a fast rate from a low engine speed condition.
Normally, the turbine of the turbocharger is sized so that high turbine speed results when the engine is operating at low speed. The flow rate of exhaust gas from the engine and passing through the turbine produces high turbine and compressor speed, high enough so that a sufficient boost in intake manifold pressure at low engine speed results. However, when engine speed increases, the flow rate of exhaust gas will increase the speed of the turbine and compressor and can produce such a large increase in intake manifold pressure that the engine could be damaged. For this reason turbochargers are often equipped with a wastegate which opens at high engine speed and bypasses a portion of the exhaust gas flow around the turbine so that the speed of the turbocharger is kept low and intake manifold pressure is maintained below a critical value. Without a wastegate in an engine equipped with a turbocharger, whose size is set to produce full boost when the engine speed is moderate, excessive engine back pressure and the associated power loss at high engine speed would necessarily result. If a wastegate is not used and the turbine is sized to produce maximum boost at low engine speed, high back pressure losses operate to reduce the efficiency of the engine system and structural damage to the engine can result.